Goran Tripun Karapetrov

Charge density waves, electronic crystals that form within a host solid, have long been theorized to melt into a spatially textured electronic liquid. Although such liquid charge density waves have not been previously observed, they may be central to the phase diagrams of correlated electron systems, including high-temperature superconductors and quantum Hall states. In 1T-TaS2, a promising material for hosting a liquid charge density wave, a structural phase transition hinders observation. Here we use femtosecond light pulses to bypass this transition, revealing how topological defect dynamics govern hidden charge density wave correlations. Following photoexcitation, charge density wave diffraction peaks broaden azimuthally, indicating the emergence of a hexatic state. At elevated temperatures, photoexcitation fully destroys both translational and orientational orders, leaving only a ring of diffuse scattering—the hallmark of a liquid charge density wave. These findings offer compelling evidence for a defect-unbinding transition to a charge density wave liquid. More broadly, this approach demonstrates a route to uncover electronic phases obscured by intervening transitions in thermal equilibrium.

Transition metal dichalcogenides like 2H-NbSe2 exhibit remarkable electronic properties, but their performance is highly sensitive to defects, particularly selenium vacancies. Here, we demonstrate controlled migration of Se atoms from the bulk to the surface of NbSe2 single crystal, induced by heat, electron beam irradiation, and laser excitation. Using Raman spectroscopy, atomic force microscopy, and energy-dispersive x-ray spectroscopy, we identify the formation of nanosized (200–500 nm) amorphous selenium clusters on the surface, evidenced by a distinct Raman mode at 250 cm−1. Temperature-dependent studies reveal the thermally activated nature of this process, with an activation energy of 1.12 eV—significantly higher than in related dichalocogenide 1T-TiSe2, suggesting suppressed Se diffusion in NbSe2 at elevated temperatures. The results provide an estimate of the fabrication thermal budget for future electronic and optoelectronic devices utilizing NbSe2.

Cryogenic calorimeters are among the leading technologies for searching for rare events. The CUPID experiment is exploiting this technology to deploy a tonne-scale detector to search for neutrinoless double-beta decay of 100Mo. The CUPID collaboration proposed an innovative approach to assembling cryogenic calorimeters in a stacked configuration, held in position solely by gravity. This gravity-based assembly method is unprecedented in the field of cryogenic calorimeters and offers several advantages, including relaxed mechanical tolerances and simplified construction. To assess and optimize its performance, we constructed a medium-scale prototype hosting 28 Li2 MoO4 crystals and 30 Ge light detectors, both operated as cryogenic calorimeters at the Laboratori Nazionali del Gran Sasso (Italy). Despite an unexpected excess of noise in the light detectors, the results of this test proved (i) a thermal stability better than ±0.5 mK at 10 mK, (ii) a good energy resolution of Li2 MoO4 cryogenic calorimeters, (6.6 ± 2.2) keV FWHM at 2615 keV, and (iii) a Li2 MoO4 light yield measured by the closest light detector of 0.36 keV/MeV, sufficient to guarantee the particle identification requested by CUPID.

We report measurements of anisotropic triple-q charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1T-TiSe2 over a large volume of reciprocal space with x-ray diffuse scattering. Above the transition temperature, TCDW, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. In addition, the out-of-plane diffuse scattering is characterized by rodlike structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations with a hierarchy of length scales, leading to intricate fluctuation patterns that go beyond the conventional 2D-to-3D crossover picture.

Superconducting transition-edge sensors (TES) have emerged as fascinating devices to detect broadband electromagnetic radiation with low thermal noise. The advent of metallic transition metal dichalcogenides, such as NbSe2, has also created an impetus to understand their low-temperature properties, including superconductivity. Interestingly, NbSe2-based sensor within the TES framework remains unexplored. In this work, direct-probed superconducting NbSe2 absorbers led to a proof-of-concept demonstration for the transduction of incoming light to heat, where a thermodynamic superconducting phase transition in NbSe2 was evident to switch it to the normal-state, when biased below its superconducting transition temperature. A wavelength-dependent response of its optical absorption properties was observed, based on the incident optical excitation source used. Furthermore, extensive optical characterization studies were conducted using Raman spectroscopy, where the in-plane and out-of-plane thermal conductivity was empirically determined. Our results open possibilities for the use of NbSe2 in superconducting radiation detectors, including in a TES framework. [Display omitted] •Incoming optical radiation absorbed by superconducting NbSe2•Heat generated from absorption causes switching to normal-state•Switching characteristics depend on wavelength of incoming radiation•Superconducting NbSe2 shows promise for bolometers and radiation detectors

We present a design and implementation of a frequency-tunable superconducting resonator. The resonance frequency tunability is achieved by flux-coupling a superconducting LC loop to a current-biased feedline; the resulting screening current leads to a change of the kinetic inductance and shift in the resonance frequency. The thin-film aluminum resonator consists of an interdigitated capacitor and thin line inductors forming a closed superconducting loop. The magnetic flux from the nearby niobium current feedline induces Meissner shielding currents in the resonator loop leading to a change in the kinetic part of the total inductance of the resonator. We demonstrate continuous frequency tuning within 160 MHz around the resonant frequency of 2.7 GHz. We show that: (1) frequency up-conversion is achieved when a kilohertz ac modulation signal is superimposed onto the dc bias resulting in sidebands to the resonator tone; (2) three-wave mixing is attained by parametrically pumping the nonlinear kinetic inductance using a strong rf pump signal in the feedline. The simple architecture is amenable to large-array multiplexing and on-chip integration with other circuit components. The concept could be applied in flux magnetometers, up-converters, and parametric amplifiers operating above 4 K when alternative high-critical-temperature material with high kinetic inductance is used.

Materials with high magnetoelectric coupling are attractive for use in engineered multiferroic heterostructures with applications such as ultra-low power magnetic sensors, parametric inductors, and non-volatile random-access memory devices. Iron–cobalt alloys exhibit both high magnetostriction and high saturation magnetization that are required for achieving significantly higher magnetoelectric coupling. We report on sputter-deposited (Fe0.5Co0.5)1−xHfx (x = 0 – 0.14) alloy thin films and the beneficial influence of Hafnium alloying on the magnetic and magnetostrictive properties. We found that co-sputtering Hf results in the realization of the peening mechanism that drives film stress from highly tensile to slightly compressive. Scanning electron microscopy and x-ray diffraction along with vibrating sample magnetometry show reduction in coercivity with Hf alloying that is correlated with reduced grain size and low film stress. We demonstrate a crossover from tensile to compressive stress at x ∼ 0.09 while maintaining a high magnetostriction of 50 ppm and a low coercive field of 1.1 Oe. These characteristics appear to be related to the amorphous nature of the film at higher Hf alloying.

CUPID is a next-generation tonne-scale bolometric neutrinoless double beta decay experiment that will probe the Majorana nature of neutrinos and discover lepton number violation in case of observation of this singular process. CUPID will be built on experience, expertise and lessons learned in CUORE and will be installed in the current CUORE infra-structure in the Gran Sasso underground laboratory. The CUPID detector technology, successfully tested in the CUPID-Mo experiment, is based on scintillating bolometers of Li2MoO4 enriched in the isotope of interest Mo-100. In order to achieve its ambitious science goals, the CUPID collaboration aims to reduce the backgrounds in the region of interest by a factor 100 with respect to CUORE. This performance will be achieved by introducing the high efficient alpha/ beta discrimination demonstrated by the CUPID-0 and CUPID-Mo experiments, and using a high transition energy double beta decay nucleus such as Mo-100 to minimize the impact of the gamma background. CUPID will consist of about 1500 hybrid heat-light detectors for a total isotope mass of 250 kg. The CUPID scientific reach is supported by a detailed and safe background model based on CUORE, CUPID-Mo and CUPID-0 results. The required performances have already been demonstrated and will be presented.

CUPID will be a next generation experiment searching for the neutrinoless double beta decay, whose discovery would establish the Majorana nature of the neutrino. Based on the experience achieved with the CUORE experiment, presently taking data at LNOS, CUPID aims to reach a background free environment by means of scintillating (Li2MoO4)-Mo-100 crystals coupled to light detectors. Indeed, the simultaneous heat and light detection allows us to reject the dominant background of alpha particles, as proven by the CUPID-0 and CUPID-Mo demonstrators. In this work we present the results of the first test of the CUPID baseline module. In particular, we propose a new optimized detector structure and light sensors design to enhance the engineering and the light collection, respectively. We characterized the heat detectors, achieving an energy resolution of (5.9 +/- 0.2) keV FWHM at the Q-value of Mo-100 (about 3034 keV). We studied the light collection of the baseline CUPID design with respect to an alternative configuration which features gravity-assisted light detectors' mounting. In both cases we obtained an improvement in the light collection with respect to past measures and we validated the particle identification capability of the detector, which ensures an a particle rejection higher than 99.9%, fully satisfying the requirements for CUPID.